Radiolabelled dihydropyridines were used as a probe to monitor for material with high affinity for dihydropyridine recognition sites. Material with an apparent molecular weight of 1100 daltons was isolated from whole rat brain, and was purified by HPLC technique. Studies on 3-H-nitrendipine binding in saturating conditions in the absence of the presence of this endogenous modulator showed that it was not competing for dihydropyridine recognition sites but indicated an allosteric modulation of these sites. The endogenous modulator inhibited reversibly calcium accumulation elicited by veratridine in cerebellar granule cells in tissue culture, and consequently blocked increase of cyclic GMP formation and protein kinase C activation in these cells. The role of the endogenous modulator on calcium channel activity was further substantiated in neuroblastoma cells (NIE-115) using whole cell patch clamp technique. Both low and high threshold types of calcium channels that occur in these cells were suppressed by the endogenous modulator at all potentials tested.